Almost all semiconductor devices include at least one high conductivity contact region in the body of semiconductor material to which an ohmic contact is made. Such contact regions are generally formed by imbedding a high concentration of ions of a desired conductivity type into the semiconductor body. A technique for imbedding the ions into the semiconductor body which has come into general use is the well-known technique of ion implantation followed by an annealing, which activates the ions and removes any damage in the semiconductor body caused by the implantation. For certain types of semiconductor devices, such as MOS field effect transistors, the implantation of the ions is carried out through a thin layer of silicon oxide on the surface of the semiconductor body. However, even where there is no silicon oxide layer intentionally provided on the surface of the semiconductor body, there is generally at least a thin layer of oxide on the surface which is formed when the material of the semiconductor body is exposed to the oxygen in the ambient.
During the implantation process some of the oxygen atoms are knocked-out of their positions in the silicon oxide matrix by the high energy ions speeding through the oxide layer into the semiconductor body. Some of the oxygen "knock-on" atoms become imbedded in the semiconductor body. These "knock-on" atoms can cause damage to the semiconductor body and decreased implant activation during the implant annealing step, which can cause increased contact resistance to an overlying metal interconnect layer in the completed device.
In order to eliminate the oxygen knock-on atoms it would appear that all that would be needed would be to eliminate the oxide layer and implant directly into the surface of the semiconductor body. However, as stated above, silicon, the major semiconductor material used, readily oxidizes when exposed to the oyxgen in the ambient resulting in a thin layer, of about 30 angstroms thick, of native oxide. It is believed that most of the oxygen knock-on atoms which end up in the semiconductor body come from regions only a few atomic layers from the silicon oxide-silicon interface. Thus, even a thin native oxide layer provides an oxygen source for knock-on into the semiconductor body which is equivalent to a layer of silicon oxide 300 to 500 angstroms in thickness.
One possible way of eliminating oxygen knock-ons would be to use a layer of a material which contains little or no oxygen and which itself would be electrically innocuous if it was knocked-on into the semiconductor body. For a silicon semiconductor body, amorphous or polycrystalline silicon would meet this criteria. However, whatever material is used, it must be removed, wholly or partially, to permit the contact material to be applied directly to the surface of the contact region in the semiconductor body. Thus, using the same material as the semiconductor body is not completely acceptable since it would be difficult to selectively remove.
Another material which has been suggested for this purpose in U.S. Pat. No. 4,030,942 to W. A. Keenan et al., issued June 21, 1977 entitled "Semiconductor Masking For Device Fabrication Utilizing Ion Implanatation And Other Methods", is aluminum nitride which is deposited on the semiconductor body by a chemical vapor deposition process. However, when using such a process the semiconductor body will probably be exposed to the ambient while placing it in the deposition apparatus. As previously stated this will result in a thin layer of native oxide being formed on the surface of the semiconductor body which provides sufficient knock-on oxygen atoms to cause problems. Therefore, it is desirable to have a method by which a layer of a material having little or no oxygen therein and which is innocuous in the material of the semiconductor body can be applied to the semiconductor body so as to permit ion implantation without the adverse effects of oxygen knock-on.